Preheat the oven to 350 degrees F. and line a muffin pan with 12 muffin liners. Set aside.
In a large bowl, whisk together 1 ½ cups all-purpose flour, ⅔ cups plus 2 tablespoons granulated sugar, 1 ½ teaspoons baking powder, ¼ teaspoon ground cinnamon, and ¼ teaspoon salt.
In a separate medium mixing bowl, combine the 2 cups mashed bananas (mash bananas with a fork or potato masher), 1 large egg, 8 tablespoons melted butter and ¼ cup milk until fully incorporated.
Stir the banana mixture into the large bowl with the dry flour mixture. Stir with a wooden spoon just until the ingredients are combined, but do not over-mix as it could result in a tough muffin.
Carefully stir in the 4 ounces of chopped semi-sweet chocolate until combined.
Add the mixture into the prepared muffin pan. Divide the batter among the 12 prepared muffin cups, filling each about ¾ full.
Bake the muffins until the tops are pale golden and a tester inserted into the center comes out with some melted chocolate attached but no crumbs, about 25 to 30 minutes.
When done, carefully transfer the muffins to a rack to cool for about 10 to 15 minutes, then completely remove the muffins from the muffin pan. Enjoy!
The chocolate banana muffins can be stored in an airtight container at room temperature for up to 4 days.